The present invention relates to a semiconductor integrated circuit having redundant portions, and particularly to a semiconductor memory device having a redundant scheme.
Memory capacity of semiconductor memory devices has been remarkably increasing. In accordance with the increase in memory capacity, a number of circuit elements included in each memory chip and an area of each memory chip are increased. Accordingly, the possibility that at least one defective memory cell is present in a memory chip has become large. In order to save such a memory chip having at least one defective memory cell, a redundant scheme has been introduced. According to the redundant scheme, redundant memory cells are fabricated on the same chip on which a normal array of memory cells are formed. In the case where a defective cell or cells are present in the normal array, such defective memory cells are functionally replaced with redundant memory cells. Thus, even the memory chip has defective memory cells in the normal array, the memory chip can utilized as a functionally good one.
In such a semiconductor memory device incorporating a redundant scheme, it is important to know whether or not the redundant scheme is actually used for replacing defects in the normal array, in view of quality and reliability controls. One approach is to provide a read-only memory (ROM) of a fuse type which indicates whether the redundant scheme is actually used to replace defects in the normal array. For example, the U.S. Pat. No. 4,480,199 proposes such technology. According to this U.S. Patent, a voltage switch circuit and a fuse are connected in series between a power voltage terminal (Vcc) and one of external terminals. The fuse is blown-out in the case where the redundant scheme is used, and is maintained conductive when the redundant scheme is not used. The programmed state of the fuse is detected by applying a special voltage outside the normal voltage range for the memory device to the one external terminal thereby to make the voltage switch circuit conductive. Under this condition, when the fuse is kept conductive, a certain amount of current flows from the one of external terminals to the power voltage terminal. On the contrary, in the case where the fuse is blown-out, no current flows between the above two terminals even when the special voltage is applied to the one external terminal.
However, this technique requires the special voltage which is outside the normal voltage range. Therefore, read process of the state of the fuse is relatively difficult. Furthermore, because of the application of the above special voltage to the one external terminal, an internal circuit connected to this one external terminal is likely to be affected by the special voltage and hence there is caused the possibility that an abnormal current flows into the above internal circuit. In the worst case, elements of the internal circuits would be destroyed by the special voltage.